The invention relates generally to an apparatus and method for an optical communications system, and in particular, to an analog frequency modulated (FM) optical receiver.
Communication systems based on optical technologies are becoming more common due to advantages over conventional wire-based communication systems. Although digital optical links can provide high data bandwidths, in some implementations analog optical links are preferred, for example where providing digital processing capability at the transmitter is impractical.
An analog optical link can exhibit an unacceptable noise figure (NF) and an unacceptable spurious-signal-free dynamic range (SFDR). For example, conventional Mach-Zehnder modulated optical links are subject in general to second order harmonics and third order difference intermodulation products. Thus a need exists for a simple and inexpensive high performance analog optical link having a low noise figure and a high SFDR.
The present invention relates to an apparatus and method for all-optical, frequency modulated (FM) communication. The method and apparatus make use of a balanced receiver configuration having a set of optical filters. An optical beam splitter provides an optical signal to two distinct optical filters. One optical filter removes the upper sideband and the other optical filter removes the lower sideband. The filtered signals are detected and their photocurrents subtracted before the resulting electrical signal is provided to a differentiator for demodulation. Laser relative intensity noise (RIN) and second order harmonics are thereby eliminated. In addition, third order distortion is eliminated when no intensity modulation (IM) is present, or greatly reduced when IM is present, As a result, this analog optical link has a low noise figure and a high spurious-signal-free dynamic range.
The present invention features a transmitter for an optical communication system which includes a FM optical signal source and an optical filter in communication with the FM source. The optical filter produces a single sideband optical signal. In one embodiment the transmitter also includes a beamsplitter and a second optical filter. The beamsplitter is in optical communication with the FM optical signal source and has a first and second beam output. Each optical filter is in communication with a respective beam output and removes one sideband of the FM optical signal.
The invention also features a receiver for an optical communication system which includes an optical filter, a photodetector in optical communication with the filter, and a differentiator in electrical communication with the photodetector. The optical filter removes one sideband of a FM optical signal to produce a single sideband FM optical signal and the differentiator produces a demodulated electrical signal in response to the single sideband FM optical signal. In one embodiment the receiver also includes a beamsplitter and a second optical filter. The beamsplitter is in optical communication with the FM optical signal source and has a first and second beam output. Each optical filter is in communication with a respective beam output and produces a respective single sideband FM optical signal in response to the FM optical signal.
The invention also features a communication system which includes a FM optical signal source, an optical filter in optical communication with the FM optical signal source, a photodetector in optical communication with the filter and a differentiator in electrical communication with the photodetector. The differentiator produces a demodulated FM electrical signal in response to the FM optical signal. In one embodiment the system also includes a beam splitter, a second filter and a second photodetector in electrical communication with the differentiator. The differentiator produces a demodulated electrical signal in response to the detected electrical signals from the photodetectors.